Page 43 - Vitamin D and Cancer

30 J. Thorne and M.J. Campbell

adjacent to this domain is the Serine 51 residue. This residue appears crucial for
ligand-induced and phosphorylation-dependent transcriptional activation by the
VDR. When Ser51 is mutated, phosphorylation of the VDR, by PKC at least, is all
but completely abolished and its transcriptional activity is markedly reduced [67].
It is intriguing that the crucial site of PKC activity is located so close to the DNA-
binding domain, but whether there are allosteric or biochemical changes that alter
the ability of the VDR to bind DNA remains to be elucidated.
The common NR partner RXR can also be phosphorylated and as a result alters
recruitment of cofactors to its holo-complexes. Ser260 is located within the ligand-
binding domain of the RXR and appears crucial for mediating cofactor binding and
ligand-induced transcriptional responses. When phosphorylated, Ser260 allows
binding between the RXR and VDR, but presumably through allosteric changes to
the complex, limits the recruitment of cofactors to the complex [68].
The recruitment of cofactors to the VDR holo-complex also appears to be regu-
lated further by the presence of PTMs, for example, kinase CK-II. The phospho-
mimic mutant VDRS208D does not increase or decrease VDR–DNA, VDR–RXR,
or VDR–SRC interactions but it does increase the levels of VDR–DRIP205 com-
plexes present. CK-II which specifically phosphorylates Ser208 enhances
1,25(OH) D -induced transactivation of VDR targets [69, 70]. In addition, phos-
2 3
phatase inhibitors (okadoic acid) in combination with 1,25(OH) D shifts the cofac-
2 3
tor preference from NCOA2/GRIP-1 to TRIP2/DRIP205 [71]. Taken together,
these data suggest that the TRIP2/DRIP205 coactivator complex enhances the tran-
scriptional response by VDR and is recruited by CK-II dependent phosphorylation
of the VDR at Ser208.

2.1.3 Vitamin D Response Elements

A further level of specificity may arise from the specificity of binding sequence
contained within the REs sequences of genomic targets. Simple REs are formed by
two recognition motives and their relative distance and orientation contributes to
receptor-binding specificity. Thus, the first identified VDRE was the DR3 – an
imperfect hexameric direct repeat sequence AGTTCA with a spacer of three nucle-
otides. In the DR3 configuration, RXR, the heterodimer partner is believed to
occupy the upstream half-site and VDR the downstream motif with two half-sites
spaced by three nucleotides. Other types of VDREs have since been identified. One
such VDRE is a palindromic sequence with a nine base-pair nucleotide spacer
(IR9). This sequence was identified in the human calbindin D9K gene and like most
VDREs the VDR/RXR binds this sequence in a 5¢-RXR-VDR-3¢ polarity (reviewed
in [72]). More recently, a novel everted repeat sequence with a six base-pair nucle-
otide spacer (ER6) has been identified in the gene for CYP3A4 (an enzyme impor-
tant in xenobiotic metabolism) in addition to the DR3 already known to be present
in this gene [73]. An inverted repeat with no spacer (IR0) has also been identified
in the SULT2A1 gene [74].